This invention relates to a method for reducing the amount of anionic metal-ligand complex in a solution containing the complex. The invention also relates to a method for removing metal-ligand complexes from waste water to make the water environmentally more acceptable. Particularly, the invention relates to a method for removing substantially all metal-cyanide, metal-thiocyanate, metal-thiosulfate, metal-citrate and/or metal-ethylenediamine tetraacetic acid (EDTA) complex from a solution containing one or more of the same.
Cyanides have been used for many years in such industries as metal electroplating, electroless plating and the recovery of precious metals from ores. As a result of such industrial uses, substantial amounts of waste water containing complexes of metal-cyanides, or their related cyanate or thiocyanate ionic species, are generated daily. Gold mining, alone, accounts for a large proportion of contaminated waste water streams.
Through the years, several methods have been proposed for removing metal-cyanide complexes from industrial solutions. Although these methods may be operable, they are also usually accompanied by serious practical or commercial disadvantages, including high operating costs, complicated equipment requirements or careful processing controls of the kind which are not customarily available at remote mine sites or at many water treatment facilities. In U.S. Pat. No. 4,092,154, there is disclosed a method for precipitating precious metal ions from a metal-cyanide-containing solution. The method proceeds by adding to the solution an admixture of aluminum powder and reducing agent selected from alkali metal hydrosulfite, alkali metal borohydride and a hydrazine. Alternatively, U.S. Pat. No. 4,537,686 teaches a method for lowering the total cyanide content of an aqueous effluent by treating the waste water at a pH between about 5-12 with an oxygen-containing gas, copper catalyst and reagent selected from: SO.sub.2 and an alkali, or alkali earth metal, sulfite or bisulfite.
The silver complex found in most photographic processing solutions is known to exist as a stable, soluble silver-thiosulfate complex. The complex typically forms by the reaction of silver bromide with ammonium thiosulfate during photograph development. The most frequently used methods for removing silver-thiosulfates from solution include: (a) chemical replacement cartridges wherein iron replaces silver when the solution is passed through stainless steel wool; and (b) conventional ion exchange resins. In U.S. Pat. No. 4,394,354, it is further taught that activated carbon which has been impregnated with at least about 0.1 wt. % halogen acts as an adequate silver ion remover. Similar improvements may be realized by impregnating halogens into such non-carbon adsorbents as natural or synthetic zeolites, activated alumina, activated silica, Fuller's earth, bentonite clay and hectorite clay, according to U.S. Pat. No. 4,396,585.
For some time now, it has become important to remove silver from waste water streams for health and environmental reasons. The subsequent recovery of precious metals has also increased in economic importance. It is well known that ionic silver can be recovered from photographic processing solutions by the electrolytic methods disclosed in U.S. Pat. Nos. 4,026,784, 4,111,766 and 4,166,781. Typically, such recovery methods were only used for solutions containing greater than 500 ppm silver since it was extremely difficult to reduce silver concentrations below about 500 ppm by electrolysis. Other disadvantages with these methods include the high expenditures and continuous monitoring requirements for such systems.
Citric acid and ethylenediamine tetraacetic acid (EDTA) have been used as chelating agents for many years in metal electroplating, electroless plating, various wood or pulp treatments and as a scale remover/controller for equipment cleaning. As a result, metal-citrate and metal-EDTA complexes often abound in the waste waters of industries employing such chelating agents, chelators or ligands. Metal-citrate complexes also exist in the water runoffs of areas where the ground is rich in citric acid deposits. Several known processes for removing and recovering from solution metal complexes of this variety are discussed in an article by Spearot et al entitled "Recovery Process for Complexed Copper-Bearing Rinse Waters", Environmental Progress, Vol. 3, no. 2, pp. 124-128 (1984). The disadvantages associated with respective recovery processes are also discussed therein. Elsewhere, U.S. Pat. No. 4,157,434 discloses a method for removing metal-EDTA complexes from a polyphenylene ether resin solution. The method includes contacting the solution with alumina to selectively adsorb the metal-EDTA thereon.
In a Chibwe et al article entitled "Intercalation of Organic and Inorganic Anions into Layered Double Hydroxides", Journal of Chemical Society, Chemical Communications,No. 14, pp. 926-927 (July 15, 1989), there is disclosed a process for intercalating several specific anions, including iron cyanide, into a hydrotalcite-like compound. The process exposes heat-treated layered double hydroxides (LDH's) to an anionic solution before centrifugation and distilled water washing.